The present invention relates to a cutting insert for use on a cutting tool such as a face milling cutter or other types of cutters.
An example of a conventionally used cutting insert is disclosed in Japanese Patent Laid-Open No. 53-101792.
Such a cutting insert is adapted for use on a vertical-edge type cutting tool. As shown in FIGS. 12 to 14, the cutting insert has a body 1 of a generally diamond-shaped tabular form having two end surfaces 2, 2 which oppose each other across the thickness of the tabular form. Four side or peripheral surfaces 3 are perpendicular to the end surfaces 2, 2. Main cutting edges 4 are formed on four peripheral edges of alternate end surfaces as viewed in the circumferential direction of the body 1, among the peripheral edges where both end surfaces 2,2 meet the respective side surfaces 3.
Numeral 5 denotes corner edges formed by two adjacent side surfaces 3,3 at the acute angle corners C, C of the body 1 of the cutting insert. A tapered surface 6 is formed at a corner of the side surface 3 which is defined by each edge 5 and the adjacent peripheral edge which does not form the main cutting edge 4. The width of the tapered surface 6 progressively increases towards the corner edge 5. The edge or side of tapered surface 6 merging in the associated corner edge 5 presents a sub-cutting edge 7 which is connected to the main cutting edge 4 forming an obtuse angle therebetween. A rake surface 8 is presented by the side surface 3 which is designed on one hand by the corner edge 5 associated with the sub-cutting edge 7 and on the other hand by the peripheral edge where the main cutting edge 4, in such manner that the rake surface 8 is connected to the main cutting edge 4 and the sub-cutting edge 7.
Numeral 9 designates a nose portion formed in the region where the main cutting edge 4 and the sub-cutting edge 7 merge with each other. Numeral 10 denotes a chamfered portion formed along the peripheral edges of the body 1 where the main cutting edges 4 are not formed, while 11 designates a mounting hole for receiving a clamp screw by means of which the body 1 of the cutting insert is mounted on, for example, a face milling cutter.
More specifically, the cutting insert is mounted on the outer peripheral portion of an end of a face milling cutter 12 shown in FIG. 15, such that one of the end surfaces 2, 2 is directed outwardly of the cutter, with one of the main cutting edges 4 on this end surface 2 projecting outward from the end of the cutter 12, as well as the sub-cutting edge 7 associated with main cutting edge 4. In this state, the body 1 of the cutting insert is fixed by the clamp screw to the cutter 12 so as to be subjected to cutting operation. The cutting insert, when used on the cutter, is so oriented that the above-mentioned rake surface 8, connected to the main cutting edge 4 and the sub-cutting edge 7, is directed forwardly as viewed in the direction of rotation of the cutter which is indicated by white-blank arrow T, shown in FIG. 15. More particularly, the rake surface 8 is a negative rake surface which orthogonally intersects the end surface 2 connected to the above-mentioned main cutting edge 4. This end surface 2 presents a relief surface connected to the above-mentioned main cutting edge 4. At the same time, the tapered surface 3 formed on the side surface 3 directed to the end of the cutter 12 presents a relief surface outward from the end of the cutter 12, as well as the sub-cutting edge 7 associated with main cutting edge 4. In this state, the body 1 of the cutting insert is fixed by the clamp screw to the cutter 12 so as to be subjected to cutting operation. The cutting insert, when used on the cutter, is so oriented that the above-mentioned rake surface 8, connected to the main cutting edge 4 and the sub-cutting edge 7, is directed forwardly as viewed in the direction of the rotation of the cutter which is indicated by white-blank arrow T, shown in FIG. 15. More particularly, the rake surface 8 is a negative rake surface which orthogonally intersects the end surface 2 connected to the above-mentioned main cutting edge 4. This end surface 2 presents a relief surface connected to the above-mentioned main cutting edge 4. At the same time, the tapered surface 3 formed on the side surface 3 directed to the end of the cutter 12 presents a relief surface which is connected to the above-mentioned sub-cutting edge 7.
In these Figures, a reference numeral 14 designates a tip pocket which is formed in a portion of the outer peripheral surface of the cutter 12 which is ahead of the rake surface 8 of the cutting insert as viewed in the direction of the rotation.
As stated before, this known cutting insert has a diamond-shaped tabular body 1 which is provided with cutting edges 4 on the peripheral edges of the alternate end surfaces 2, and sub-cutting edges 7 are formed so as to be connected to the main cutting edges 4 along the corner edges 5 between the adjacent peripheral surfaces 3, 3 intersecting at both acute corners C, C of the body 1. Therefore, by turning the body 1 through 180.degree. from the illustrated position or by reversing the same, such that the other end surface 2 is exposed, it is possible to use the cutting insert in four different postures, one after another.
It has been known that, in this type of cutting insert, the cutting resistance imposed on the main cutting edge 4 is effectively reduced to improve the cutting performance, by setting large positive axial and radial rake angles of the main cutting edge 4 in the mounted state of the cutting insert.
However, in the known cutting insert of the type described, the peripheral surface 3, on which the rake surface 8 of the main cutting edge 4 is formed, is a planar surface which orthogonally intersects the end surface 2 associated with the main cutting edge 4, over the entire length of the peripheral edge of the end surface 2. Therefore, in order to obtain a large axial rake angle of the main cutting edge 4 while preserving the relief of the sub-cutting edge 5, the apex angle at the acute angle corners C, C, i.e., the angle at which the side surfaces 3, 3; 3, 3 forming the respective corners C, C, is set to a considerably small value. Such a reduced acute angle causes the body 1 of the cutting insert to have a flattened diamond shape, with the result that the nose angle of the sub-cutting edge 7 is reduced, so that the strength of the cutting edge is impaired, thus allowing a problem to occur, such as breakdown.
On the other hand, a cutting insert also has been known in which, in order to preserve a large radial rake angle of the main cutting edge 4, the body 1 of the cutting insert is so designed that the end surface 2 adjacent to this main cutting edge 4 and the side surface 3 intersect at an angle which is below 90.degree., so that the rake surface 8 connected to this main cutting edge 4 is formed as a positive rake surface which intersects the above-mentioned end surface 2 at an acute angle. In this type of cutting insert, however, the nose angle of the main cutting edge 4 is reduced, so that the strength of the main cutting edge is reduced, thus posing the risk of breakdown of the main cutting edge.
In particular, the above-mentioned risk of breakdown of the cutting edge is critical in the case where the cutting insert is used on a face milling cutter described above, because in such a case the cutting insert receives the greatest load at its cutting edge which is projected beyond the end of the cutter 12.
If the mounting of the cutting insert on the face milling cutter 12 is conducted in such a manner as to impart a positive axial rake angle to the main cutting edge 4 while giving an outer peripheral relief angle to the end surface 2 directed to the outside of the cutter 12 as shown in FIG. 15, the distance between the axis of rotation the cutter 12 and the main cutting edge 4 varies according to the portions of the cutting edge 4, so that both ends of the main cutting edge 4 slightly project radially outward of the cutter 12 as compared with the central portion of the main cutting edge 4. Consequently, the locus of the main cutting edge 4 formed when the cutter 22 rotates is not truly cylindrical but is slightly constricted at the center. As a consequence, the finish precision of the surface cut by the face milling cutter is impaired.
Since the amount of projection of both ends of the main cutting edge 4, outward from the cutter 12, is different from that of the central portion of the main cutting edge 4, different levels of load are applied to both ends and the central portion of the main cutting edge 4. In the operation of the face milling cutter having the described cutting insert, the metal chips formed as a result of the cutting performed by the main cutting edge 4 are caused to slide on the rake surface 8 formed on the side surface of the body 1 of the cutting insert and is received in the aforementioned tip pocket 14 so as to be discharged therefrom.
However, in the known cutting insert, the main cutting edge is essentially straight because it is formed along the peripheral edge formed between the end surface 2 and the side surface 3, both of which are planar. Consequently, the cut chips of the metal also are liable to have a wide web-like form with a large curling radius, tending to cause a jam or clog with the cut metal chips. Such a web-like cut metal chip is hardly separable from the planar rake surface 8 along which the metal chip advances is tight contact therewith. This promotes the wear of the rake surface 8 and enhances the cutting resistance.